The present invention relates generally to a method of B0 controlled shimming of a magnet assembly of an MR system and, more particularly, to a method of shimming the magnet assembly of an MR system to achieve a near-homogeneous magnetic field having requisite signal strength without requiring mechanical adjustments to the magnet assembly once assembled.
It is generally known that when a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or xe2x80x9clongitudinal magnetizationxe2x80x9d, Mz, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
During fabrication and construction of the magnet assembly for an MR assembly, manufacturing tolerances and deviations in material make-up of the magnet assembly result in an inhomogeneous B0 field being created by the magnet assembly absent shimming. As a result of the magnet manufacturing process, it is not uncommon for the magnet to produce a very inhomogeneous field ranging from several hundred parts per million (ppm) to several thousand ppm, and a non-accurate center magnetic field that is significantly out of range. The importance of these variations is glaringly apparent given that MR systems require an intense uniform magnetic field, typically less than 10 ppm of variations within a 40-50 cm spherical volume, but also an accurate center magnetic field value, typically less than 0.5% variation.
Shimming is a common process that is used to remove inhomogeneities from the B0 field. Shimming is important for MR systems because the average B0 field strength must be within a certain window for the RF hardware of the system. A simplistic example of the effects of shimming is graphically shown in FIG. 1. As shown, a magnet assembly without shimming produces a magnet field represented by curve 2. The variations of the magnetic field are quite clear. As is widely known, these variations negatively affect data acquisition and reconstruction of an MR image. As such, it is desirable to generate a shim field, represented by curve 4, that counters or offsets the variations in the magnetic field. The combination of the shim field 4 with the magnetic field 2 yields, ideally, a homogeneous and uniform B0 field represented by curve 6.
The shimming process includes the precise placement of shim elements within the magnetic assembly such that numerous small magnetic fields are generated to offset variations in the B0 field. The shim elements include active shims such as shim coils or permanent magnets as well as passive shims such as iron cores. Shim coils are common in superconducting magnet assemblies and their shimming may be controlled by regulating current thereto. The shimming characteristics of permanent magnets may be controlled by regulating the mass and polarity of the magnet and the shimming effect of iron cores may be controlled by regulating the mass of the iron incorporated into the magnet assembly.
Regardless of the type of shim element employed, the customary manufacturing and shimming process measures the B0 field of the magnet assembly and then shims the magnet assembly with precise placement of shim elements. The placement of shim elements, however, is done without regard to the affects the shims have on the average field strength of the center B0 field. That is, shimming is concerned with homogeneities in the field and mechanical adjustments to the magnet assembly are later done independently of to address issues regarding average field strength. For example, in a permanent magnet MRI system, mechanical adjustments may include changing the air gap between yokes of the magnet assembly. However, these mechanical adjustments may deviate sufficiently from the magnet design such that performance characteristics such as fringe field are adversely affected. Moreover, mechanical variations or adjustments to the magnet assembly are a time-consuming and costly process that often requires several iterations before sufficient shimming has occurred. In fact, it is not uncommon for the shimming process to take several days to complete. Other approaches include implementation of a mechanical device within the magnet assembly to increase or decrease the B0 field. This device is generally referred to as a xe2x80x9cB0 plugxe2x80x9d and it increases the overall weight, size, and cost of the magnet assembly.
It would therefore be desirable to have a system and method capable of prescribing shimming of an MR magnet assembly such that time consuming and costly mechanical variations to the magnet assembly are avoided. It would also be desirable to design a model wherein peak-to-peak homogeneity and central field issues are addressed simultaneously.
The present invention overcomes the aforementioned drawbacks by providing a method of shimming a magnet assembly of an MR imaging system such that a desired B0 field strength may be created with minimal inhomogeneities therethrough. With this method, sufficient shimming of the magnet assembly may be achieved without requiring mechanical variations to the magnet assembly after the magnet assembly has been assembled. The method, which may be carried out as a set of instructions of a computer program by one or more computers, analyzes variations from the desired B0 field and inhomogeneities at a number of target points along the magnet assembly or B0 field. A comparison is then made at each point to determine a shimming or weighting factor such that the desired B0 field strength and targeted field homogeneity are achieved during data acquisition. Active and/or passive shim elements may then be incorporated into the magnet assembly at each target point to achieve the desired overall field strength and minimum overall field homogeneity. The shimming or weighting factors are used to determine the amount of xe2x80x9cshimming materialxe2x80x9d that is used at each target point.
xe2x80x9cShimming materialxe2x80x9d varies according to the type of shim element. For example, for active shim elements, i.e. shim coils, the shimming material corresponds to the amount of current applied to the coils. By varying the amount of current applied to the coils, the amount contributed to the magnetic field can be varied. As a result, the shim coils can be independently controlled such that field contribution is precisely controlled. For passive shim elements, i.e. iron shims or permanent magnets, the shimming material corresponds to the amount of magnetic element that is added to the magnet assembly.
Therefore, in accordance with one aspect of the present invention, a method of shimming a magnet for an MR imaging system includes the steps of determining a desired B0 field strength for a B0 field having a number of target points and determining a minimum acceptable field inhomogeneity for the B0 field. The method also includes the step of determining at least one of a field strength variation from the desired B0 field strength and an inhomogeneity variation from the minimum acceptable field inhomogeneity at each target point. Each target point of the B0 field is then shimmed such that at least one of an actual B0 field strength at least approximates the desired B0 field strength and an actual field inhomogeneity does not exceed the minimum acceptable field inhomogeneity.
In accordance with another aspect of the present invention, a computer readable storage medium having a computer program stored thereon to develop a shimming model for a magnet assembly of an MR imaging system, the computer program representing a set of instructions that when executed by a computer causes the computer to map a B0 field generated by an assembled magnet assembly. From the map, a number of target points within the B0 field or, alternately, a set of harmonics are identified. The set of instructions then causes the computer to determine an amount of shimming required at each of the target points such that a desired field strength of the B0 field is maintained simultaneously with substantial cancellation of inhomogeneities within the B0 field.
According to another aspect of the invention, a method of manufacturing a magnet assembly for an MR imaging system is provided. The method includes the step of constructing a permanent magnet assembly designed to generate a B0 field at a desired field strength about a volume-of-interest (VOI). Variations in field strength along the B0 field are then determined. Variations in the field strength are then minimized without requiring mechanical adjustments to the permanent magnet assembly.
Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings.